Phase modulation recording and reproducing system



y 30, 1944- w. VAN B. ROBERTS 6 PHASE MODULATION RECORDING ANDREPRODUCING SYSTEM Filed Jan. 30, 1942 2 Sheets-Sheet 1 7 6 Phase, 6"Mnrogkone No a 5 Er I I QIKc.

. Man 70' g -ay r INVENTOR Wag aroBJ2oZerb ASI'TORNEY ay 1944- w. VAN B.ROBERTS PHASE MODULATION RECORDING AND REPRODUCING SYSTEM Filed Jan. 50,1942 2 Sheets-Sheet 2 IA'ILI'ORNEY Patented May 30, 1944 UNITED sTATEsPATENT OFFICE PHASE MODULATION RECORDING AND EEPRODUCING SYSTEM Waltervan B. Roberts, Princeton, N. 1., assignor to Radio Corporation ofDelaware of America, a corporation Application January to. 1942, SerialNo. 428,810

15 Claims.

My present invention relates to recording and reproducing systemswherein the modulation exists on the record as a timing modulation of acarrier wave, and more particularly to systems of the aforesaid typewherein the audible signals are caused to phase modulate the carrierwave.

In my application, Serial No. 369,829, flied December 12, 1940,"I havedisclosed and claimed re cording and record-reproducing systems whereintiming modulated carrier waves are provided on the record. Specifically,the audible signals frequency modulate a super-audible carrier wave, andthe modulated waves are recorded on the record. Various advantages areoutlined in the aforesaid application for such systems. entirelyindependently of the general advantages of timingm'cdulation inrecording systems there are certain special advantages to be secured in.phase modulation (PM) of a recorded carrier wave over phase modulationof a radio wave. To

appreciate these special advantages. consider first v an ordinary phasemodulation radio system. In

such a system a single carrier is phase modulated and radiated.

The generation and transmission of such, a

wave involves no ditllculty, but the demodulation of the wave is by nomeans easy. This demodulation is accomplished ,by' deriving, from thephase modulated carrier, an unmodulated carrier and then combining thisunmodulated carrier with the phase modulated wave in suitable phaserelation so that the vector sum of the two varies in amplitude as themodulated component varies in phase. The variable amplitude vector sumdifficult part of the above process is obtaining the unmodulated carrierwhich acts as a voltage of reference phase. This step is made relativelyeasy when a phase, modulation recording system is involved by recording,in accordance with the 40 present invention, both a phase modulated waveand also an unmodulated wave of different but integrallyrelatedfrequency. In demodulating, these two waves are separated fromeach other by selective, circuits and then by the process 01-"45frequencymultiplication or division or both, are brought to a-commonfrequency. The relative phase is then properly adjusted an the/twowaves, now at the same frequency'andof suitable phase relation, arecombined to form a single 5 variable amplitude vector sum which isrectified as previously described in connection with radio systems. Itwill be seen that in the recording system the required unmodulated wavehaving a frequency always absolutely identical with the 5 Now, 15

mean frequency of the modulated wave and having an unvarying phaserelation thereto, is much more easily obtained.

Accordingly, it may be stated to be one of the main objects of mypresent invention to provide,

"in a system of recorded signals, a method wherein a pair of waves ofintegrally related frequencies are applied to a record-producing member,one of the waves being super-audible and being subjected to phasemodulation by signals to be recorded.

Another important, object of my invention is to combine a phasemodulated carrier wave and a reference wave in harmonic relation as aphysical record; and reproducing the record by separating the referencewave from the modulated wave, and phase comparing the separated wavesafter frequency multiplication of the reference wave to and there beingutilized phase comparing means for the aforesaid output waves thereby toprovide for the demodulation of the phase modulated waves. voltage isthen rectified to produce signals. The

Still other objects of my invention are generally to improve theefficiency and reliability of recording and record-reproducing systems,and more especially to provide systems utilizing phase modulatedsuper-audible carrier waves for recording purposes in aneconomically-manufacturable manner.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by, reference to the following description takenin connection with the drawings in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

. In the drawings:

Figure 1 shows schematically a recording sys-- tem employing theinvention;

Figure 2 shows a modification network of th system of Figure 1,

Figure 3 shows schematically a record-reproducing system employing theinvention;

Figure 4 shows a modified system that can be used in place of thereproducing system of Figure 3.

Referring now to th accompanying drawings, wherein like referencecharacters in the different figures designate similar circuitielements,it is to be understood that numeral I designates the source of audiosignals to be recorded. It will, of course; be understood that while theaudio source is shown as a microphone, it may well be any other desiredaudio signal source. For example, it may be the audio output of thedetector of a radio broadcast receiver, or it may be an audio frequencydistribution line, or it may even be the reproducer of anotherphonograph. The numeral 2 designates the turntable upon which issupported the record3 on whose upper face the recording is to be out. Amotor 4, of any desired and well known type, rotates the turntable 2;the spindle I passing through an opening in the record 3 so as toprevent slippage.

These are devices well known to those skilled in theart of recording. Itis to be understood, of course, that the record i'mayuse a base eitherof metal, fibre composition or glass. The cutting-of the record can beof any desired type commonly employed in the art of recording. It will,also, be understood that either embossing or engraving may be employedfor the production of the lateral recording. Specifically, there isdisclosed an engraving cutter. While the cutter has been schematicallyrepresented as one of the electromagnetic type, it will be understood,of course, that a crystal cutting head may be utilized, as disclosed inmy aforesaid co-pending application.

Of. course, if a crystal cutting head is employed, then the windings 1and 8 might be replaced by a pair of crystal actuating electrodes, and acommon grounded electrode, one actuating electrode being connected to 6'while the other actuating electrode is connected to lead It). It will,therefore, be appreciated that the specific manner of cutting therecord, and the cutting devices utilized; may be of any well knowntype.- The cutting stylus 9 may be of any desirable and well known type.Reference is, also, made to my aforesaid application wherein there aredescribed various methods of providing the recording proper on therecord.

Referring, now, to the method of translating the audio signals fromsource I into physical displacements of the cutting device, there isshown in Figure l a master oscillator III which is operated at apredetermined frequency, as, for example, 'l kilocycles (kc.). Themaster oscillator frequency has a predetermined relation to thesuper-audible carrier wave frequency which is to be directly modulatedby the audio signals. The oscillations from oscillator M are transmittedover conductor l directly to the winding 8 whose lower end is grounded.These oscillations, which are unmodulated and of constant amplitude,provide the reference wave.

Oscillations from master oscillator I0 are concurrently applied to afrequency multiplier net- The phase modulator 6 has applied to it audiosignals from source I, and the third harmonic oscillations from themaster oscillator. The phase modulator produces, at its output, phasemodulated carrier waves whose mean frequency is 21 kc. The PM waves aretransmitted over a conductor 6' to winding l of the cutting device, thelower end of winding 1 being grounded. It will, therefore, be seen thatthe stylus 9 is actuated by virtue of the energization of windings I and8, and, therefore, there will be provided a recording which is acomposite of the reference waves fed to winding 8 and the PM waves fedto winding I.

The master oscillator ill, the frequency multiplier ll an dphasemodulator network 6 have all been schematically represented since theyare networks which are well known to those skilled in the art. The phasemodulator network may be of any well known type, and reference is madeto Communication By Phase Modulation" by M. G. Crosby, proceedings ofthe I. R. E.; February 1939; pages 126 to 136, for

various types of phase modulators that may be employed. It will now beseen that two waves have-been superposed on the record 3; one of thesewaves is unmodulated and of constant am; plitude, while the other wavehas a mean frequency, in harmonic relation to the reference wavefrequency, and is phase modulated in accordance with the audio signalswhich may be voice or music.

Of course, the unmodulated reference wave and the superaudible carrierwave may both be derived by frequency multiplication from a single lowfrequency wave. As shown in Figure 2, the master oscillator, by way ofillustration, is operated at a low frequency of 2 kc. The low frequencyoscillations are fed to a frequency multiplier network l2. For example,a frequency multiplication of four times may be utilized therebyproviding an 8 kc. reference wave to the lead Ill. The output of thefrequency multiplier [2 may be supplied to an additional frequencymultiplier analogous to multiplier H of Figure 1 so that thesuper-audible carrier wave for modulation is provided.

The record now being made, the recording is readily reproduced by thesystem shown in Figure 3. In this figure the record 3, or a duplicatethereof, is placed upon a turntable 2' cperated by motor 4'. The pick-updevice is schematically represented by numeral l3. Here, again, whilethe pick-up is shown as an electromagnetic pick-up, it is to be clearlyunderstood that a crystal pick-up device may be used to transform themechanical vibrations into electrical energy of a phase modulatedsuper-audible frequency voltage whose amplitude is more or lessconstant.

However, as shown in Figure 3, after suflicient amplification it ispreferred that the modulated waves be passed through a limiter stage forremoving substantially all amplitude modulation. While a constantamplitude has been referred to heretofore, what is mean is that noattempt is made to vary the amplitude. As a matter of fact, even if thecutting stylus or reproducing network has an inherent tendency tooperate more efilciently at one frequency than at another, it will makelittle or no difference in the final result because the limiter networksused in Figure 3 nullify the effect of amplitude variations.

Considering the circuit of Figure 3 more spe- 'quency-is 21 kc. The passband of the selector network l should be wide enough to pass with highefilciency the entire spectrum of the super audible carrier wave.

The selector it, since it is transmitting the unmodulated referencewave, may be sharply tuned to 7 kc., which is the frequency of thereference wave. For example, the selector it may be a piezo-electriccrystal tuned to 7 kc. This is of especial value, as shown in Fig. 5 ofmy copending application serial No. 431,320 filed Feb.

18, 1942, where automatic means are provided for keeping the turntablespeed sufficiently constant. The outputs of selector networks I! and i6may then be passed through the limiter stages i5 and i6 respectively.The function of the limiter stages is to suppress all amplitudevariations, and thus prevent any amplitude variations in the respectivechannels from affecting the PM detector tubes. Those skilled in the artare fully aware of the construction of the limiter stages. Above thelimiter I! there is shown the characteristic of a limiter stage. It willbe seen that'the output rises proportionally with input up to apredetermined amplitude, and beyond that the curve flattens off., Sincelimiter devices are very well known to those skilled in the art, it isnot believed necessary to include any further description thereof.

1 The output of limiter i5 is applied to the primary winding oftransformer 11 which feeds the PM carrier wave to the detector. As shownin Fig. 3, the secondary winding l'l' has its opposite ends connected bydirect current blocking condensers i8 and I8 to the anodes 0f theoppositely. connected diodes II and II. The cathodes of the two diodesare connected in common, and the common cathode connection to themidpoint of input coil il' includes the secondary winding 20 oftransformer II which feeds the nowsynchronous reference wave into thedetector circult. The primary winding of transformer ii is connected inthe output circuit of a phase adjuster device 22, the input terminals ofthe phase adjuster being fed with the frequency multiplied output oflimiter It.

The frequency multiplier 23 multiplies the 7 kc. reference wave to 21kc., since the reference wave must be injected into the PM detectorcircuit at the same frequency as the mean carrier frequency of the PMcarrier wave. The phase adjuster 22, which may be of any type well knownto those skilled in the art, is adjusted so that in the absence of anymodulation on the superaudible carrier wave the voltage components oneach of diodes I! and I! are in quadrature. In other words, the phaseadjuster 22 phases the reference wave voltage fed into winding 20 sothat the reference wave voltage is in quadrature with the PM wavevoltage applied to the anode of each of the diodes when there is'nomodulation.

Another way of considering the phase relations between the PM voltageand the reference wave voltage is to consider that the PM wave voltageis in one quadrature phase with the reference wave voltage on one of thediodes, while it is in the opposite quadrature phase relation on theother diode. Of course, these phase relations are considered for thesuper-audible carrier wave in unmodulated condition compared to theunmodulated reference wave. Accordingly, when modulation exists on thesuper-audible carrier. wave, the detector outputs yield a differentialvoltage which corresponds to the original audio modulation signals. Tosecure this differential modulation voltage, a resistor 24 is conectedbetween the anodes of diodes i9 and IS. The midpoint of the resistor isconnected to the common cathode connection of the two diodes. Thedifferential voltage developed across resistor 24 is applied through theaudio coupling condensers to one or i more stages of audio frequencyamplification.

The phase modulation detector circuit is purely illustrative, as thereare other types of phase modulation detector circuits that can beutilized. Reference is made to the aforesaid Crosby publication to showsuch other types of phase modulation detector circuits. The function ofthe injection of the frequency multiplied reference wave into the inputcircuit of the detector circuit is to convert the applied PM carrierwave into an amplitude modulated wave. As explained previously, thistranslation of the PM carrier wave into an AM carrier wave isaccomplished by rotating the carrier vector degrees with respect to themodulation side bands. Injecting the multiplied reference wave as shownprovides a standard of phase against which to compare the phase of thePM carrier wave. The rectifiers i9 and IQ, of course, function toprovide differential rectification of the AM wave, which results fromcombining the unmodulated reference wave with the PM wave. It is notbelieved necessary to describe the functioning of the phase modulationdetector circuit in any further detail, as those skilled in the art arewell acquainted with the manner of demodulating a PM carrier wave.

While I have described a particular embodiment of the invention in Fig.3, it will be realized that considerable variation is possible withinthe scope of the invention. For example, the unmodulated reference wavevoltage may be applied differentially to the diodes, and the phase modulation carrier voltage may be applied in like phase relation. Again, ifthe amount of phase swing recorded is too little, or too great, for thevoltage comparing and detecting network, then the phase swing may beincreased by frequency multiplication, or decreased by frequencydivision in the channel which feeds the phase modulated carrier wave tothe detector. In any case there must always, of course, be suchfrequency multiplication or frequency division of the unmodulatedcarrier wave or reference wave, or both, as to provide components at thedetector of identical frequency.

Thus, in Fig. 4 there is shown a modified arrangement which embodies theaforesaid modifications. It will be seen that the selector i5 feedsfrequency divider 30 which divides the mean frequency of the PM carrierwaves by three (3),

and thereby also proportionally decreases the phase deviation. The 7 kc.waves are fed through the limiter-l5 and a frequency doubler ll to thephase adjuster 22, and the adjusted PM waves are applied to the winding.20 whereby the PM carrier waves are now applied in like phase to thedifferentially connected diodes. The selector II separates out thereference waves of 7 kc., and

after limiting at I6, these waves are frequency multiplied at 23 toprovide a reference wave frequency of 14 kc. The reference waves areapplied to the diodes of the detector in differential manner.

Of course, in Fig. 4 there is illustrated the situation where the phaseswing recorded is too great for the voltage comparing and detectionnetwork, and, hence, frequency division is employed as at 30. Where thephase swing is too small, frequency multiplication will be utilized at30 instead of frequency division.

While I have indicated and described several systems for carrying myinvention into eil'ect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. In a system for recording signals, means for generating a pair ofwaves of integrally related frequencies, means to phase modulate one ofsaid waves, means to combine the remaining unmodulated wave and thephase modulated wave, and means to translate the combined waves into aphysical record.

2. In a system for recording signals, means for generating a pair ofwaves of integrally related frequencies, means to phase modulate one ofsaid waves, means to combine the remaining unmodulated wave and thephase modulated wave, means to translate the combined waves into aphysical record, and said phase modulated wave being of super-audiblefrequency and bearing an harmonic reiationto the unmodulated wave.

3. In a reproducing system for a record upon which is recorded anunmodulated reference wave and a phase modulated wave of super-audiblefrequency which is integrally related to the frequency of theunmodulated wave, means to derive said waves from the record, selectivemeans to separate the waves into separate channels, frequency changingmeans in at least one of said channels adapted to bring the output wavesof said channels to a common frequency, and phase comparing means forsaid waves including means to derive the modulation on the wave ofsuper-audible frequency.

4. A method of recording audible signals which includes generating apair of waves of integrally related frequencies, one of said waves beingof super-audible frequency, phase modulating said super-audible wave,and recording the combined modulated and unmodulated waves.

5. In a method of reproducing a record upon which has been recorded aphase modulated carrier wave and an unmodulated reference wave inharmonic relation, separating the reference wave from the modulatedwave, frequency multiplying the reference wave to the modulated wavefrequency, and phase detecting the combined modulated and multipliewaves.

6. The method of reproducing signals which comprises generating a pairof alternating currents of integrally related frequencies, phasemodulating one of said pair in accordance with said signals, recordingsaid modulated current and the other current of said pair, reproducingthe recorded currents, separating the reproduced currents into separatechannels, bringing the separated currents to a common frequency,combining the common frequency currents in nor- 16 mally quadraturephase relation, and rectifying the combined currents to reproduce saidsignals.

7. In a system for recording signals and reproducing the recordedsignals, means for generating a pair of waves of integrally relatedfrequencies, means to phase modulate one of said Waves with the signals,means to combine the remaining unmodulated wave and the phase modulatedwave, means to translate the combined waves into a physical record,means to convert the record to said waves, means for separating thewaves, means to bring both wave to a common frequency, and phasedetecting the latter waves.

8. In a reproducing system for a record upon which is recorded anunmodulated reference wave and a phase modulated wave of super-audiblefrequency which is integrally related to the frequency of theunmodulated wave, means to pick up said waves, selective means toseparate the wave into separate channels, frequency multiplier means inat least one of said channels adapted to bring the output waves of saidchannels to a common frequency, phase comparing means for said waves,and means coupled to the comparing means to derive the modulation on thewave of super-audible frequency.

9. In a method of reproducing a record upon which has been recorded aphase modulated carrier wave and an unmodulated reference wave inharmonic relation, separating the reference wave from the modulatedwave, frequency multiplying the reference wave to the modulated wavefrequency, combining the modulated wave and multiplied frequency wave innormally quadrature phase relation, and rectifying the combined waves.

10. In a system for recording signals, means for generating a wave of adesired frequency, deriving therefrom a pair of waves of integrallyrelated frequencies, means to phase modulate the higher frequency one ofsaid waves, means to combine the remaining unmodulated wave and thephase modulated wave, and means to translate the combined waves into aphysical record.

11. In a system for recording signals, means for generating a wave oflow frequency, frequency multiplying the low frequency wave to a higherfrequency, deriving from the multiplied wave a pair of waves ofintegrally related frequencies, means to phase modulate one of said pairof waves, means to combine the remaining unmodulated wave and the phasemodulated wave, means to translate the'combined waves into a physicalrecord, and said phase modulated wave being of super-audible frequencyand bearing an harmonicrelation to the unmodulated wave.

12. In a reproducing system for a record upon which is recorded anunmodulated reference wave and a phase modulated wave of super-audiblefrequency which is integrally related to the fre quency of theunmodulated wave, electrical pickup means to pick up said waves,selective means to separate the waves into separate channels, frequencychanging means in at least one of said channels adapted to bring theoutput waves of said channels to a common frequency, means to combinethe latter output waves in normally quadrature phase relation, andopposed rectifiers coupled to the combining means for deriving themodulation signals.

13. A method of recording audible signals which includes generating asuper-audible frequency wave of constantamplitude, phase modulating saidsuper-audible wave, recording the modulated Wave, reproducing saidrecord upon which has been recorded said phase modulated carrier wave,deriving from the reproduced wave a reference wave, combining thereference and modulated wave in normally phase quadrature relation, anddetecting the combined waves.

14. A method which includes generating a super-audible frequency wave ofconstant amplitude, phase modulating said super-audible wave withaudible signals, recording the phase modulated wave, converting saidrecorded phase modulated wave, converting said recorded phase modulatedcarrier wave into corresponding modulated wave energy, deriving from themodulated wave energy a reference wave, and combining the reference andmodulated waves for phase detection.

15. A method which includes phase modulating super-audible frequencyoscillations with audio signals, translating the phase modulatedoscillations into a physical record, converting the record into phasemodulated oscillations corresponding to said original phase modulatedoscillations, producing unmodulated oscillations having a frequencyequal to the mean frequency of said converted oscillations, and phasedetecting the combination of the converted phase modulated oscillationsand saidlunmodulated oscillations in predetermined phase relation toproduce said audio signals.

WALTER wm B. ROBERTS.-

